1. Field of the Invention
The present invention relates to the generation of codes and has been developed with particular attention to the possible application for generating channeling codes that can be used for code-division-multiple-access (CDMA) communications.
2. Description of the Related Art
In communication systems there exist various techniques that enable access to a common channel for transmitting information according to typical multiple-access modalities.
A traditional solution envisages dividing the available channel bandwidth into a number N of sub-channels. Each user who wishes to transmit information is thus assigned a particular sub-channel. This technique is generally referred to as frequency-division multiple access (FDMA).
Another technique operates, instead, in the time domain, so that the above-mentioned division operation for enabling multiple access is performed by dividing a time interval Tf, commonly referred to as time frame, into a plurality of N sub-intervals which do not overlap one another, each having a duration of Tf/N. Each user who wishes to transmit information is thus assigned a particular time slot in the context of each frame. This technique is generally referred to as time-division multiple access (TDMA).
A further technique, which is alternative with respect to the above-mentioned FDMA and TDMA techniques, envisages enabling a number of users to share a certain channel or sub-channel by using spread-spectrum (SS) signals. With the adoption of this solution, each user is assigned an encoded channeling sequence, which is unique for each user. This sequence enables the user to distribute, or spread, the information signal over the frequency band assigned to him. In this way, the signals coming from different users can be separated at the receiver by cross-correlation between the signal received and each of the possible channeling codes assigned to the various users. If the said encoding operation is performed in such a way as to have relatively small cross-relations, it is possible to minimize the crosstalk during demodulation of the signals received from different transmitters. This multiple-access technique goes by the name of code-division multiple access (CDMA).
In CDMA applications, the users access the channel in a random way, so that the various signal transmissions are prone to overlapping completely both in time and in frequency. In the receiver, the operation of separation demodulation of these signals is rendered possible in so far as the signal is spread in frequency through the channeling code.
For example, with reference, for simplicity of illustration, to the case in which four users are present, it is possible to implement a CDMA multiple-access scheme using four channeling codes, each comprising four binary figures. Each user is thus able to distribute, or spread, his signal, which leads to the formation of four spread-spectrum signals, which are found, in any case, to be mutually orthogonal when superimposed to form a CDMA signal. At the receiver, the regenerated composite signal is received, and the signal corresponding to each user can be separated from the others by exploiting, precisely, the orthogonality of the corresponding encodings.
The foregoing corresponds to criteria that are well known in the state of the art and, as such, do not require any more detailed description herein.
In brief, a transmitter operating in a CDMA system uses the channeling codes for sharing the common propagation channel. On the other hand, the receiver (Terminal Equipment, or briefly TE) must be in a condition to generate all the channeling codes used so as to demodulate the signals received by the various transmitters, separating them from one another.
If the transmission speeds currently used are considered (for example, 3.84 MHz in the Universal Mobile Telecommunication System (UMTS) standard), the said speed being moreover bound to increase over time, it is important to be able to generate the codes in question in a simple and fast way, without giving rise to excessively high levels of power consumption, above all taking into account the need to operate in the context of mobile terminals.
The present invention has been developed with particular attention paid to its possible application to two classes of channeling codes currently referred to as the Walsh-Hadamard (in brief, WH) code and the Orthogonal-Variable-Spreading-Factor (OVSF) code.
For general information on the WH codes, useful reference may be made to J. Proakis, “Digital Communications,” McGraw-Hill, p. 422 et seq. and Roger L. Peterson, “Introduction to Spread Spectrum Communication,” Prentice Hall, p. 542 et seq.
For general information on the OVSF codes and the UMTS standard, reference may be made to Standard 3G TS 25.213 V3.2.0 UMTS Standard Document, Release 2000-03.
The invention can, however, be applied in general to all the codes that present the same characteristics as the WH and OVSF codes, to which reference will be made in what follows.
The particular attention paid to these two types of channeling codes is due to the fact that they have been chosen in view of their possible utilization in the framework of the UMTS standard referred to previously.
As further clarification, FIG. 1 is a comparative illustration of the characteristics of the WH channeling code (FIG. 1A) and the OVSF channeling code (FIG. 1B) for values of length L equal to 2, 4 and 8, respectively.
In general both the WH codes and the OVSF codes may be viewed as a vector function of two variables, namely:
the length (L), and
the index (I=0 at L−1).
The length is in general a power of 2 (i.e., 2n, with n integer), and each code may be viewed as corresponding to the i-the row of a corresponding WH/OVSF matrix in which each element of the matrix is an antipodal binary number (i.e., ±1).
In the solutions up to now proposed for the generation of codes, such as the WH and OVSF codes, the generation of the codes themselves is envisaged in an altogether independent way.
It happens, on the other hand, that the generation of the WH code is simpler (and hence less burdensome in terms of circuit complexity), requiring typically—for instance, for a code of length L=8—a circuit complexity in the region of 200 gates.
Generation of the OVSF code is in general more burdensome. For example, there has recently been proposed a solution whereby, to generate an OVSF code of length L=8, a circuit having a complexity in the region of 400 gates is used.
In certain applications there arises, however, the need to generate both codes.
For example, with reference to the UMTS application already mentioned more than once previously, the US standards (for instance, the standard IS95CDMA) envisage the use of WH codes, whereas in the European context the use of OVSF codes has prevailed for the very same application.
In order to create systems, in particular systems of mobile terminals, that are able to operate with different standards, it is therefore important to have available solutions that enable generation of both codes in a simple and fast way, reducing energy consumption to a minimum, at the same time avoiding the need to resort to a solution of a purely additive nature, based upon the use of a first generator for producing WH codes and a second generator (distinct from the first) for producing OVSF codes; i.e., a solution which, with reference to the orders of circuit complexity discussed previously, would entail the use of circuits having a circuit complexity in the region of 600 logic gates.